drm/amdgpu/si: implement get/set pcie_lanes asic callback

Required for dpm setup on some asics. Fixes a NULL dereference
on asics that require it.

Acked-by: Christian König <christian.koenig@amd.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=102553


Tested-by: Abel Garcia Dorta <mercuriete@yahoo.es>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org